The invention relates to a method and an apparatus for measuring an unknown quantity of heat in a measuring zone.
So-called differential calorimeters are known to be calorimeters of high measuring precision. They comprise two completely similar vessels which contain measuring zones which have the same heat losses. While the unknown quantity of heat to be measured is supplied to one vessel, the second vessel is heated electrically by introducing a known measurable quantity of heat in such a manner that no temperature differences can be detected between the two vessels. Under these conditions, the two quantities of heat supplied are then identical. It is also possible to heat with a known quantity of heat only a single vessel which contains a single measuring zone, to ascertain the heat loss of the vessel, and then to carry out measurement with the unknown quantity of heat.
With both methods, there is a lack of precision in the measuring operations as a result of heat exchange with the environment and the like.
The invention has as its object to overcome this disadvantage. For this purpose a method for measuring an unknown quantity of heat in a measuring zone is carried out in such a manner according to the invention that the measuring zone is supplied with a known quantity of heat and the quantity of heat of the measuring zone is given off to a cold zone, that the temperature of the measuring zone is always kept equal to the temperature of the environment and at the same time the dissipation of heat is kept constant by keeping the difference between the temperature of the measuring zone and the temperature of the cold zone always the same, and that the known quantity of heat supplied to the measuring zone is measured when the unknown quantity of heat is absent and when it is present, the difference between the known quantity of heat in the absence of the unknown quantity of heat on the one hand and the known quantity of heat in the presence of the unknown quantity of heat on the other hand being the measurement result for the unknown quantity of heat.
An apparatus for carrying out this method with a vessel heat-insulated relative to the environment for receiving an unknown heat source is characterized according to the invention in that the vessel comprises a known heat source, that a cold source is provided which is connected with the vessel by way of a heat-conducting element, that heat sensors are provided one for each of the environments, the vessel and the cold source, and that an electrical control device is provided which controls the known heat source and the cold source through the agency of the heat sensors in such a manner that the temperature of the vessel is always equal to the temperature of the environment and at the same time the difference between the temperature of the vessel and the temperature of the cold source is always the same.
According to one particular constructional form of the apparatus, the cold source comprises an additional heat source which is so controlled by the control device that the difference between the temperature of the environment and the temperature of the cold source is always the same.
According to one particular constructional form of the apparatus, the known heat source is a transistor since to reach the possible accuracy of the calorimeter, the heat supplied to the vessel by the heater has to be measured with the same accuracy. With a transistor used as a linear heater in the present invention, the collector current (or the collector-emitter voltage) is exactly proportional to the heat production, if the collector-emitter voltage (or the collector current) is kept constant. Therefore, the electronical integration of the power yielding the total heat supplied the vessel within a given time can be performed easily with a high degree of accuracy. Also, the use of a squaring device, which has to be used in prior art arrangements such as in U.S. Pat. Nos. 3,813,937 and 2,982,132, and which limit the accuracy, can be avoided.
According to a further special constructional form of the apparatus, the cold source consists of Peltier elements.
The drawings show a constructional example of an apparatus for carrying out the method according to the present invention in simplified form, by means of which the method according to the invention will also be explained.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, several embodiments in accordance with the present invention.